With smoke from the Galena Fire hanging over Fort Collins, health officials are warning residents to avoid exercising outdoors until the air clears.

“In general, if you live in an area that is experiencing smoky conditions, it is recommended that you avoid outdoor physical activities,” the Larimer County Department of Health and Environment said in a statement. “If visibility is decreased in your neighborhood to less than 5 miles, smoke has reached levels that are unhealthy.”

Experts say smoke from a wildfire can be particularly unhealthy due to the small-size particles it carries. Ash drifted Friday onto Fort Collins’ western neighborhoods, and many mountain bikers who were riding in the Horsetooth Reservoir area were turning around rather than ride into smoke blanketing the area.

The health department said kids, the elderly, pregnant women and people with respiratory conditions are especially vulnerable. Experts said if those people can see or smell smoke, they should stay inside with the windows and doors closed.

When smoke levels are high, even healthy people may experience coughing, a scratchy throat, irritated sinuses, shortness of breath, chest pain, headaches, stinging eyes and a runny nose, the health department said.